Hydromechanical load-splitting transmissions are known, as for example from my commonly owned U.S. patent application 967,006 filed Dec. 6, 1978 and from commonly owned patent application 896,501 filed Apr. 17, 1978. In these arrangements the force applied to the input shaft of the drive is split up and passed in part through a mechanical transmission, and in part through a hydrostatic transmission. The mechanical transmission may have several gear ranges. The hydrostatic transmission normally has a variable-volume pump that is driven by the input shaft of the drive and a fixed-volume motor that in turn is connected to an input element of a summing transmission having another input element connected either through a gear box or directly to the input shaft of the drive.
Such a drive is capable of producing a relatively high output torque and speed, using hydrostatic elements which are of substantially smaller capacity than would be necessary to produce this output torque and speed all alone. Thus the main advantage of hydrostatic elements, namely the stepless variation in speed is obtained without the high cost normally associated with such arrangements.
Other devices are known, as for example from Austrian Pat. No. 265,796 of Oct. 25, 1968, which produce a steplessly variable output speed using a fixed-speed electric motor at the input. Here the use of hydrostatic elements once again makes it possible to achieve this stepless variation in output speed while at the same time producing a drive of relatively low cost.
All of these systems have the considerable disadvantage, however, that it is impossible to achieve a relatively high output speed with them, even at low torque. The hydrostatic elements are not capable of turning at the high speed necessary to produce a high-speed output, even though it is a relatively simple job to use gearing in the mechanical leg of the force-transmission path which is capable of producing such speeds. A high output speed is useful, for example, in a crane when lifting an unloaded grab to the upper part of the boom or in a piece of construction equipment that must travel at relatively high speed along the highway.